crossflow exhaust manifold...bubbleback vs flatback?

[ryguy1101]

Rebuilding a 1989 johnson 110. Hoping to replace the factory bubbleback housing and rotted/warped filler block assembly with the flatback assembly I have from another V4 crossflow engine b/c it's in better shape and saves me $150 or more in new filler block parts. Anyone know if I need to change carbs or other parts to match, or if I'm just asking for trouble by swapping the exhaust in the first place? Not worried about horsepower at all b/c it's going on a 17 aquasport and the boat will move really well no matter what. Fuel efficiency would actually be a higher priority considering fuel prices. I already have a couple different size carbs to choose from so a carb swap is doable. Hoping to tackle the assembly this weekend so any thoughts are welcome.

 

[emdsapmgr]

Re: crossflow exhaust manifold...bubbleback vs flatback?

If you are just swapping the exhaust filler blocks and the exhaust housings to your 110, you should be ok to run the original 110 carbs. Keep in mind when you do swap the inner blocks, you will have to have a machine shop make a pass on the exhaust deck. It all has to be perfectly flat to mate to the the inner exhaust manifold. If you are considering swapping the entire block, you may get into carb jetting issues- if that other block has high compression heads.

 

[iwombat]

Re: crossflow exhaust manifold...bubbleback vs flatback?

You can also run the bubble-back w/o the filler blocks. Once you target that x-flow to run around 6k for ethanol fuel, I think they run better w/o them anyway.

 

[ryguy1101]

Re: crossflow exhaust manifold...bubbleback vs flatback?

Hmmm...didn't know you could leave them out of the exhaust housing but just ran across another thread that seemed to say the same thing. That would be the ideal solution if no harm can come of it. My bubbleback cover and manifold are in great condition, and I have all the gaskets, it's just the fillers that are all warped and rotted. What advantage do they provide in the first place? Do they just improve aerodynamic flow through the exhaust chamber?

 

[ryguy1101]

Re: crossflow exhaust manifold...bubbleback vs flatback?

...or can I just reinstall the warped fillers? They're not too terrible, just a little out of alignment.

 

[iwombat]

Re: crossflow exhaust manifold...bubbleback vs flatback?

Theoretical Portion:
Hi-perf 2-stroke exhausts are designed to reflect a pressure wave back into the cylinder on the downstroke. This stops fuel from bypassing the cylinder and dumping into the exhaust port. It also creates more pressure in the combustion chamber during ignition. Those fillers were designed for an engine running at 5k RPMs wot. With today's fuel, it's better to prop them to run close to 6k WOT.

Practical Portion:
Given the 6k RPM tuning, the bubble back exhaust is no longer optimized to reflect a pressure wave back into the exhaust port at the right time in its original configuration. Removing the blocks makes for less restriction, which the 6k RPM tuning needs more than a mis-timed pressure wave.